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Trouble with Ohms Law and some advice please


ohdearme
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Hello all

 

I have put my ohmmeter across the output of the adaptor of a small pair of Logitech speakers. The adaptor has an output of 16 volts and 1.2 amps. The reading just moves about haphazardly.

 

Would this be something to do with the magnetic action of the transformer or am I not doing this properly?

 

More to the point, I wish to buy a replacement (as it is kaput) and I fancy a "universal" one. How important is the output and how far can I stray from the manufacturers requirement?

 

Say, If I have a speakers with an adaptor of 16 volts, what kind of percentage of milliamps up or down would make a difference and how would I judge which setting? I would not like to blast my speakers into oblivion by setting it say 50% higher than the speakers can handle.

 

Regards

 

Alan

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Presuming you have the proper settings on the multimeter, the adapter being 'kaput' may account for the erratic reading.

 

On the amperage, I think that under-amperage would give less than optimal performance and that over-amperage wouldn't matter as the speakers can only draw so much current with their fixed resistance and at a given voltage.

 

This bit is about computer adapters but the same principles apply to your speakers. >> Can I use a charger that provides the same voltage but a different amperage?

Edited by Acme
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An ohm meter uses an internal battery to measure resistance.

 

If you put it across a power source it's no wonder the reading is all over the place, it's modulating according to the power used in the circuit.

 

Ohms law doesn't work that way. It's used to calculate a missing variable of either volts, amps or ohms (resistance)

 

E = Volts (volume)

I = Amps (pressure)

R = Ohms (resistance)

 

 

post-112421-0-18452900-1448166511_thumb.jpg

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Hydraulic analogies soon run into trouble because the analogy is not exact in importartant detail.

Surely it is best to avoid all hydraulic analogies and go staight for the horse's mouth.

Isn't post 3 clear enough?

Why cloud it with muddy water?

 

Incidentally if you want to talk about inductive circuits, why introduce resistance as in post3?

 

 

 

I have put my ohmmeter across the output of the adaptor of a small pair of Logitech speakers.

 

Ohdearme, please, please don't connect your multimeter to the mains adapter when set to the ohmeter setting.

This is a good way to destroy your meter, even if the adapter is not working properly.

Definitely don't connect it like that to any new adapter, even after switching the mains off as the residual power may still be enough to damage the meter.

 

Also do not connect it in current mode to the adapter.

Bang.

 

#So that leaves us with voltage mode.

Off load the adapter can be expected to produce a higher voltage than when working at rated current.

If you adapter bears a 16V rating then it could be as high as 20V off load. (But see my note about your voltage at the end of this post)

The voltage should fall to 16 on full load. (Speakers rarely present full load.)

 

Talking of loads, let me dispel the all too common misconception,

 

Your adapter rating says 16V 1.2 amps.

 

The amps are determined by the load not by the adapter.

The amps are determined by the load.

A different load will draw a different current.

So the adapter will allow the load to draw any current from 0 to 1.2 amps at 16 volts.

 

 

This is true of all normal supplies of electricity (batteries, the mains, power supply adapters)

Only special purpose supplies such as battery chargers work the other way round.

 

BTW are you sure the supply says 16 volts not 6 volts?

That is most unusual.

Edited by studiot
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Volts is to gallons as amps is to PSI.

 

Wrong.

Hello all

 

I have put my ohmmeter across the output of the adaptor of a small pair of Logitech speakers. The adaptor has an output of 16 volts and 1.2 amps. The reading just moves about haphazardly.

 

I'm not sure why you would connect an ohmeter across this. You may have damaged it (the meter and/or the power adaptor). Did you intend to measure the voltage, to see if it is really 16V? If so, you might get misleading results as the voltage may rise when their is no load.

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I have put my ohmmeter across the output of the adaptor of a small pair of Logitech speakers.

Ohmmeter should be used when device is turned off. Not when it's turned on...

 

Reverse to voltmeter and ampere meter.

 

Voltmeter/ampere meter should be used when device is turned on.

But voltmeter parallel to circuit, while ampere meter serial in circuit.

So it's pretty hard to measure current in majority of ready electronics, because you would have to disconnect their wires/paths on electronic board.

 

If you plug ampere meter parallel to circuit, the most likely you can destroy either device and multimeter. Because it's like plugging there wire, and making short circuit.

Plugging voltmeter serial in circuit, rather won't have any influence.

 

The reading just moves about haphazardly.

What a surprise.. You should not measure resistance of your transformer/adapter. It does not even have any sense. What you were interested in anyway? Resistance of wire in transformer?

 

Only voltmeter you can plug to adapter/transformer to measure voltage between electrodes.

 

Would this be something to do with the magnetic action of the transformer or am I not doing this properly?

Quite obviously.

 

More to the point, I wish to buy a replacement (as it is kaput) and I fancy a "universal" one. How important is the output and how far can I stray from the manufacturers requirement?

As long as it's 16 V, and above or equal 1.2 A it should be pretty ok.

P = I * U = 1.2 A * 16 V = ~20 Watt adapter you have.

If you will use 30 W, 40 W, with 16 V voltage, should be fine.

 

It's resistance of wire in speaker, where is electromagnet, will limit current flowing through wire.

 

Say, If I have a speakers with an adaptor of 16 volts, what kind of percentage of milliamps up or down would make a difference and how would I judge which setting? I would not like to blast my speakers into oblivion by setting it say 50% higher than the speakers can handle.

 

If you will use adapter with U=24 V, it'll be 50% higher voltage than 16 V,

You don't know what current will be flowing through whole device in reality.

It's limited by resistance of wire in electromagnet. There is probably not another stabilizer in circuit.

 

This 1.2 A from label of adapter is just maximum value this device can give. While normal working it'll be LESS.

After exceeding 1.2 A it'll be destroyed. f.e. shorting electrodes of adapter will destroy it.

Edited by Sensei
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I have put my ohmmeter across the output of the adaptor of a small pair of Logitech speakers. The adaptor has an output of 16 volts and 1.2 amps.

 

16 volts and 1.2 amps implies 20 watts.

 

For a small pair of computer speakers?

 

This would be most unusual.

 

So please ohdearme check the rating befopre we proceed further and confirm that the adapter isn't really 6 volts.

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Hello all thanks so much for your replies.

 

Sensei the power was off when I used the ohm meter.

 

Acme, thank you for the link a good one, makes it clear that the device TAKES/PULLS amps from the supply, rather than the supply pushing it in.

 

Studiot, the Logitech power supply is definitely rated 16v 1.12 amps I have also checked on line.

 

Thanks so much for the warning, I knew about the danger of a voltage setting within in a circuit but did not think of this. I am wiser now.

 

The other part of my original question, are the so called "universal" power supply/adapters ok in your opinions?

 

Best to all

 

Alan

(ohdearme)

 

 

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I see lots of reports of expensive replacements for the Logitech 16v airplay power supply.

 

http://www.amazon.co.uk/gp/customer-reviews/R1T6H6BFR7IQN5/ref=cm_cr_pr_viewpnt?ie=UTF8&ASIN=B005LNS6WK#R1T6H6BFR7IQN5

 

Can you please confirm the model number of the speaker and power supply.

 

Also is this 16volts DC or AC (some Logitech devices use an AC supply)?

 

I caution care in replacement because the word seems to be that they are not man enough for the job.

 

If we can tie down the exact requirements we can devise a better alternative.

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I know the questions been answered but you can certainly get magnetic flux interference when measuring current / volts etc Generally speaking the lower the values your trying measure the higher chance of flux interference. I saw alot of this when converting an analogue read that turned the signal into 1032 (10 bit input), even when i knew that the source of current and the exact voltage (should have been) i would get interference on the 5V reads. I did find a way of filtering the noise to an approximation (I.E i found out what the average noise would be and did an equation to balance that average out of the input). I remember it taking ages to figure out why my reads were so different and random though.

 

If you really want a powerful system (and not blow your speakers or fuse) then I'd recommend wiring a sub up to some old / aftermarket high spec speakers. My friend built a full sound system ( 2 huge speakers + sub & amp + decks (vinyl) + mixer ) for about £100. Youd probably have to wire the decks to your computer that way and let the mixer connect you to the system but its not too difficult or expensive.

 

I'd certainly not recommend buying anything new or modifying any lower spec speakers as this will only end up with them farting anyway.

 

Also i remember her telling me its not all about the wattage, as most people think watts define how good speakers are, i think she said Ohms were important maybe for sound quality (less resistant metals = more synchronised sound?), it could have been decibel related, i cant remember for sure and both the above 2 relate directly to wattage i think so that wouldn't make sense but there is some property that shows sound quality, i suppose wattage just defines how *loud* it can actually go.

Edited by DevilSolution
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It seems Studiot, that many people are not happy with Logitech. I understand your doubt now as to the voltage as someone in Maplin said that 16 volts was unusual.

 

Anyway, I have the Logitech Speaker System Z320 (two of them, they are a "table top" type). The power supply is also Logitech ModelEFso180600112CE.

 

16v, 1.12 Amp DC. The sound was lovely, I wold like to get them working again.

 

Incidentally, I note that they were made in China. I bought them from John Lewis so am not worried but there was a tv programme, Panorama I think that showed the dangers of some on line electronic stuff, coming from China, elsewhere I think, that blew up or caught fire which is why I am rather "funny" about buying on line.

 

Trust all is well with you,

 

Best

 

Alan

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OK, we have established that you need a more robust 16 volt DC power supply.

 

Since it seems to be a waste of money getting a Logitech one, you have several options.

 

1) Some older laptops have 16V DC supplies. More modern ones are in the 19 to 21 volt range. I have PM'd you a link to an IBM 4 amp one for £1.20.

Or look in a computer shop.

 

2) Some Yamaha music equipment uses 16 volt DC 2 -3 amp adapters. So look in a music shop.

 

3) A universal adapter around 2.25 - 3 amps capacity and set it to the correct voltage. However some cheap versions of these don't last any better than the Logitch variety.

 

A couple of further points.

 

Make sure you have a decent UK mains plug connection, some click together ones are poor to dangerous.

 

Make sure you have the right plug connector on the output unless you are prepared to change the one that is there.

This is a disadvantage of the universal type. the click together all purpose connectors can easily become undone or just loose.

 

Cheers

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